1. Field of the Invention
The present invention relates to a die-dried friction wafer and to a method for making a friction plate by net-shape molding a friction wafer of fiber, fillers, and resin binder in a configured mold cavity, and bonding to a steel core.
2. Background Art
The traditional processes for manufacturing friction plates involve forming a specialized paper, a laborious wet-laid sheet forming process, then impregnating the paper with a thermoset resin(s), driving off the solvent, curing the treated paper, and applying the impregnated paper to cores, generally in the form of steel plates. The process usually involves cutting annular rings or segments from the paper sheets or the treated paper sheets. Although the paper rings can easily be cut from the sheets, much paper can be wasted from the area inside and outside the rings. Moreover, when the cutting is performed on the resin impregnated papers, the offal may not be recyclable for re-manufacturing or reclaimed for other uses.
In a conventional wet laid process, a slurry of fibers, fillers and binders is laid or drawn onto a wire mesh conveyor while the water is being removed through the wire. The wire mesh conveyor is transferred over water removing stations. This process is typically performed on a standard paper machine. The resultant porous friction paper, which does not contain a resin, is then impregnated with a resin in a subsequent processing step, dried, cured, blanked, and bonded to a steel core to make the friction assembly. The paper can be blanked into annular rings or segments prior to or after the resin impregnation process. The bonding and curing operation can also occur in one step.
The use of a so called “beater add” process for making friction materials for liquid cooled and dry friction applications involves mixing a slurry of fibers, fillers, binders, friction particles, ‘beater-add’ resin(s), processing aids and friction enhancing media in a water slurry and then removing the water using suction and drying, typically performed on a paper machine. The term “beater-add” refers to the type of resin that is added in powder form to the aqueous slurry. The resin must be compatible with water. The component(s) of the resin that cause it to cure with heat must also not dissolve in the water allowing the resin to maintain it's cross-linking or curing capability after it has been dispersed in water. The advantage of beater-add is the elimination of the saturation of the paper with resin associated with the conventional wet laid friction material manufacturing process.
In the beater add process, the resin is included in the slurry. While elimination of the saturation process step is an advantage compared to the conventional process of wet-laid material saturation, it is still not a widely used process for the production of friction assemblies due to the long standing issues of manufacturing paper sheets, and the associated laborious and wasteful blanking of rings or segments from these sheets with the beater add process.
One issue is the need to dry out the beater add material sufficiently so that it may be blanked, cured and bonded to the steel core. This dry out is generally done in the dryer section of the paper machine. If the material temperature becomes too high, the cure will be prematurely advanced, having a negative impact on product performance. A major issue is the heat retained in the material as it is rolled at the end of the paper machine. This heat can begin to cure the resin and create an exothermic reaction in the roll, often resulting in a fire hazard or a roll of unusable material. The beater-add material is therefore generally not suitable for rolling onto reels or rolls as is done for the conventional wet-laid process prior to saturation.
Hence, the process for beater add materials is generally associated with cutting large sheets or pads as opposed to rolls, and subsequently blanking the annular rings or segments from these sheets. The use of sheets rather than rolls is a serious limitation on the cost effective production of friction assemblies because sheeting requires a separate machine. In addition, the offal after blanking the ring is considered hazardous waste unless the resin is first fully cured, therefore there is added expense to fully cure and/or dispose of the offal properly.
Additionally, the carry over of constituents from the beater add formulation into the water used for the slurry often limits the use of a wet-laid machine with a closed loop water system for the production of both non-resin and beater add materials on a single machine. Due to the resin in the formulation, the large amounts of process water used in paper making must generally be filtered and treated before re-use or discharge; adding to the cost of the beater-add process.